Embodiments of the present specification generally relate to a wind turbine and more specifically to systems and methods for controlling a wind turbine.
Wind turbines are growing in popularity as a means of generating energy due to the renewable nature of the energy so generated and lack of pollution. Wind turbines generally have a rotor with a plurality of blades coupled to a generator. The power extraction capability and secure operation of a wind turbine typically depends on various factors including wind speed. For example, knowledge of potential wind speeds that will impact the rotor of the wind turbine in the following few seconds may be helpful in controlling the wind turbine for optimal power extraction.
Wind speeds are typically measured by an anemometer, such as a cup anemometer. However, anemometers are incapable of predicting the potential wind speeds that will impact the rotor of the wind turbine in the imminent future, since anemometers are only capable of measuring wind speed in the immediate surrounding area. Laser radar systems (LIDARs) have been employed for measuring wind speeds and direction of wind for many years. LIDARs have been used to measure wind shear, turbulence and wake vortices in both military and civil applications. Typically, the laser radar system (LIDAR) operates by scattering radiation from natural aerosols (for example, dust, pollen, water droplets, and the like) and measuring the Doppler shift between outgoing and returning radiation. In order to measure wind speed and direction it is usual to scan the LIDAR, typically using a conical scan or multiple fixed beams to allow a wind vector to be intersected at a range of angles, thereby enabling a true (3D) velocity vector to be deduced. Other scanning patterns may also be used to determine the true velocity vector. However, the accuracy of determining the true velocity vector is dependent on knowledge regarding the direction of the LIDAR.
One advantage of LIDAR includes prediction of the potential wind speeds approaching the rotor of the wind turbine. For example, LIDARs may be used for providing wind speed measurements up to 400 m in front or ahead of the rotor of the wind turbine. Accordingly, the LIDAR may provide information regarding approaching wind speeds to a wind turbine controller in advance, thereby increasing the controller's available reaction time and allowing pitch actuation to occur in advance to mitigate wind disturbance effects. The wind turbine controller may use feed-forward control algorithms to improve load mitigation and controller performance.
Currently available LIDARs for use with wind turbines are impacted by surrounding atmospheric conditions and other factors such as blade positions. As oncoming wind approaches a wind turbine, a drop in wind speed is fluidly induced. This wind speed decrease is known as an induction, which when measured at a specific location and normalized to the free-stream wind speed is known as an induction factor. The free-stream wind speed is the speed of the undisturbed natural air flow, usually at hub height. Betz' theory suggests that an optimal wind turbine operating condition to maximize power extraction is at an induction factor of 0.33. More discussion on Betz' theory is available at https://en.wikipedia.org/wiki/Betz%27s_law and is also discussed in Betz, A. (1966) Introduction to the Theory of Flow Machines. (D. G. Randall, Trans.) Oxford: Pergamon Press. Because LIDARs have limited range, however, it may not be possible to directly measure the free-stream wind speed at an infinite distance.